Use of epoxy film adhesive with high ink compatibility and thermal oxidative stability for printhead interstitial bonding in high density printheads

ABSTRACT

An adhesive compound can include an uncured epoxy film having a curing temperature between about 80° C. and about 300° C. The uncured epoxy film can include a cresol novolac epoxy resin and a bisphenol A epoxy resin. The uncured epoxy film can have a thickness between about 0.1 mil and about 5 mil.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of and claims priority toU.S. patent application Ser. No. 14/289,153 filed May 28, 2014, thecontents of which are incorporated herein by this reference in itsentirety.

TECHNICAL FIELD

The present teachings relate to the field of ink jet printing devicesand, more particularly, to adhesive materials usable in high densitypiezoelectric ink jet print heads.

BACKGROUND

Drop on demand ink jet technology is widely used in the printingindustry. Printers using drop on demand ink jet technology can useeither thermal ink jet technology or piezoelectric technology. Eventhough they are more expensive to manufacture than thermal ink jets,piezoelectric ink jets are generally favored, for example because theycan use a wider variety of inks.

Piezoelectric ink jet print heads include an array of piezoelectricelements (i.e., piezoelectric transducers) made of lead zirconatetitanate (PZT). One process to form the array can include detachablybonding a blanket piezoelectric layer to a transfer carrier with anadhesive, and dicing the blanket piezoelectric layer to form a pluralityof individual piezoelectric elements. A plurality of dicing saw passescan be used to remove all the piezoelectric material between adjacentpiezoelectric elements to provide the correct spacing between eachpiezoelectric element.

Piezoelectric ink jet print heads can typically further include aflexible diaphragm to which the array of piezoelectric elements isattached. When a voltage is applied to a piezoelectric element,typically through electrical connection with an electrode electricallycoupled to a power source, the piezoelectric element bends or deflects,causing the diaphragm to flex which expels a quantity of ink from achamber through a nozzle. The flexing further draws ink into the chamberfrom a main ink reservoir through an opening to replace the expelledink.

The formation of ink jet printheads typically requires lamination ofmultiple layers of materials as part of their fabrication. Traditionalprinthead designs may use layers of gold-plated stainless steel sheetmetal with features that are photochemically etched and then brazedtogether to form robust structures. However, with the continued drive toimprove cost and performance, use of alternate materials and bondingprocesses may be used. While polymer layers can be used as a replacementof some sheet metal components, polymers require adhesives with suitableproperties to bond to each other and to metal layers.

For example, the adhesive must be chemically compatible with the inksused within the printhead. Further, the adhesive should have certainphysical properties that reduce printhead failures during use. Anadhesive should have a good bond strength, a low squeeze out to preventblocking of the fluid path, and should be sufficiently resistant tooxidation with elevated temperatures during use. Also, some adhesivesmay increase in weight and swell, or become less compliant and morestiff during use when exposed to certain inks and elevated temperatures,which can result in leakage of ink or other failure modes. Some of thesefailures may occur only after extended use of the printhead.

SUMMARY

The following presents a simplified summary in order to provide a basicunderstanding of some aspects of one or more embodiments of the presentteachings. This summary is not an extensive overview, nor is it intendedto identify key or critical elements of the present teachings nor todelineate the scope of the disclosure. Rather, its primary purpose ismerely to present one or more concepts in simplified form as a preludeto the detailed description presented later.

In an embodiment, an adhesive compound can include an uncured epoxy filmhaving a curing temperature between about 80° C. and about 300° C. Theuncured epoxy film can include a cresol novolac epoxy resin and abisphenol A epoxy resin. The uncured epoxy film can have a thicknessbetween about 0.1 mil and about 5 mil.

In another embodiment, there is a method. The method can include formingan epoxy adhesive by curing an adhesive compound. The adhesive compoundcan include an uncured epoxy film having a thickness between about 0.1mil and about 5 mil and a curing temperature between about 80° C. andabout 300° C. The uncured epoxy film can include a cresol novolac epoxyresin and a bisphenol A epoxy resin.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the presentteachings and together with the description, serve to explain theprinciples of the disclosure. In the figures:

FIG. 1 is a cross section of an exemplary ink jet printhead portionformed in accordance with an embodiment of the present teachings; and

FIG. 2 a perspective view of a printer including one or more printheadsin accordance with an embodiment of the present teachings.

FIG. 3 is a differential scanning calorimetry (DSC) graph of heat flow(W/g) versus temperature (° C.) for uncured adhesive film and curedadhesive film of embodiments.

FIG. 4 is a graph of storage modulus (MPa) and loss modulus (MPa) versustemperature (° C.) for adhesive film of embodiments cured at 190° C. for70 minutes, and includes tan(6) (Tan Delta) versus temperature (° C.)showing a Tg of 149° C. for cured adhesive film.

FIG. 5 is a graph of the percent weight change in a cured adhesive film(cured at 190° C. for 70 minutes) of embodiments soaked in lab air(ambient), hot air (140° C.), LancE ink (commercial solid ink), pigmentK (pigmented black solid ink), commercial UV ink (e.g., Sunjet UV ink),HP latex ink (commercial aqueous ink), hot nitrogen (140° C.) or Collinsink (commercial aqueous ink) versus time (weeks).

FIGS. 6A-6H are graphs of lap shear aging results of lap shear couponsusing stainless steel adherends with adhesive films of embodiments inthe soaking environments described for FIG. 5. The graphs show lap shearstrength (psi) versus aging duration in weeks.

FIGS. 7A-7B are graphs showing results for burst testing of curedadhesive film of embodiments built into materials burst test structures(MTS) in coarse (FIG. 7A) and fine (FIG. 7B) features and soaked inseveral aging environments, including various inks.

It should be noted that some details of the FIGS. have been simplifiedand are drawn to facilitate understanding of the present teachingsrather than to maintain strict structural accuracy, detail, and scale.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of thepresent teachings, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

As used herein, unless otherwise specified, the word “printer”encompasses any apparatus that performs a print outputting function forany purpose, such as a digital copier, bookmaking machine, facsimilemachine, a multi-function machine, electrostatographic device, etc.Unless otherwise specified, the word “polymer” encompasses any one of abroad range of carbon-based compounds formed from long-chain moleculesincluding thermoset polyimides, thermoplastics, resins, polycarbonates,epoxies, and related compounds known to the art.

Achieving reliable adhesion between many different ink jet printheadlayers and materials, particularly at the harsh environmental conditionsfound in current ink jet printhead uses, is a concern for devicemanufacturers. An embodiment of the present teachings can result in amore robust physical adhesive connection between the various laminatedlayers within a printhead, particularly with regard to resistance tochemically harsh inks such as acrylate-based ultraviolet (UV) inks andpigmented inks, which can include pigmented aqueous and/or pigmentedsolid inks, and may result in decreased stresses on the interconnectionwhich electrically couples a piezoelectric transducer to a circuit layersuch as a printed circuit board or flexible printed circuit.

Printhead structures are known in the art and include many layerslaminated together. The adhesives used for lamination must resistreaction with chemically harsh inks, bond well to surfaces of differentmaterials to prevent rupture during high-pressure printing, and hold upduring high temperature printing, for example during printing with solidinks. FIG. 1 depicts a portion of an exemplary ink jet printheadstructure 10 that may be formed using an embodiment of the presentteachings. The FIG. 1 printhead structure 10 includes a compliant wall12, an external manifold 14, and a diverter 16 attached to the externalmanifold 14 with an external manifold adhesive 18. FIG. 1 furtherdepicts a boss plate 20 attached to the diverter 16 with a diverterattach adhesive 22. In an embodiment, the compliant wall 12 can includethermoplastic polyimide, the external manifold 14 can include aluminum,and the boss plate 20 can include stainless steel. The external manifold14 can receive liquid ink (not individually depicted for simplicity)during use which has been melted from solid ink blocks, a gel ink, a UVink, or another liquid ink in preparation for printing, and maintain theink at a print temperature. FIG. 1 further depicts a body 32, a verticalinlet 34, a separator 36, a particulate filter (rock screen) layer 38including a rock screen 40, a front end manifold 42, and an apertureplate 44 having a nozzle 46. The aperture plate 44 can be attached tothe front end manifold 42 with an aperture plate adhesive 48. In anembodiment, the body 32, the separator 36, and the front end manifold 42can include a metal such as stainless steel, and the vertical inlet 34,the rock screen layer 38, the aperture plate adhesive 48, and theaperture plate 44 can each include one or more polymers. The assembly 10can be manufactured according to known processing techniques, such as aprocess including the use of a stack press under high pressure. FIG. 1further depicts a substrate 52 such as a semiconductor wafer section,glass layer, metal layer, etc., a standoff layer 54, a printheaddiaphragm (membrane) 56, a boss plate adhesive 70, a diaphragm adhesive72, an application specific integrated circuit (ASIC) 58 attached to thesemiconductor wafer section, and an interconnect layer 60 such as aflexible (flex) circuit or printed circuit board electrically coupled tothe ASIC 58. As discussed above, the substrate 52 can be a silicon,gallium arsenide, metal, glass, etc. Further, the standoff layer 54 canbe silicon dioxide and/or SU-8 photoresist. The diaphragm 56 can be ametal such as titanium, nickel, or a metal alloy. The substrate 52 mayinclude a circuit pattern. It will be appreciated that the depiction ofthe FIG. 1 is a small portion of a printhead depicting a single ink port74 and nozzle 46, and that other structures may be added or existingstructures may be removed or modified. A printhead with current designsmay have four ink inlets, one for each color (cyan, magenta, yellow, andblack in a CMYK color model, for example), and 7040 nozzles. Thestructure of FIG. 1 may be formed using an embodiment of the presentteachings and may include a structure in accordance with an embodimentthe present teachings.

A desirable adhesive for printhead applications would be able to bondany combination of metal layers (e.g., stainless steel, aluminum, etc.)and/or polyimide layers. In selecting an adhesive, similar formulationsmay have differing properties and operating characteristics. Extensivein-house testing is required to characterize the properties of anadhesive to determine whether it has the necessary characteristics for aspecific use. While a supplier may publish some operatingcharacteristics, other unknown characteristics may be of particularinterest to a manufacturer searching for a suitable adhesive and thuscharacterization of the adhesive by the manufacturer is necessary. Alarge number of adhesive formulations are commercially available andidentifying an adhesive that has the necessary characteristics oftenpresents a formidable challenge. Further complicating the selection isthe fact that an adhesive may embody different characteristics atdifferent thicknesses, different application processes, and at differenttemperatures. Additionally, an adhesive may react differently whenexposed to different chemicals having similar formulations, for exampleto similar but different ink formulations. The variety of combinationsof epoxy resins and curing agents provides wide latitude in chemical andmechanical properties at the final cured stage.

An embodiment of the present teachings can include the use of anadhesive for physically attaching together two or more printhead parts.In use, the adhesive may be subjected to harsh chemical inks, such aspigmented inks (including pigmented aqueous inks and/or pigmented solidinks) and UV gel inks and to high temperatures and pressures associatedwith printing, for example, solid inks. In an embodiment, the adhesivemay be an epoxy-based adhesive that is formed from an adhesive compoundsuch as a thermal setting polymer. The adhesive compound can be anuncured epoxy film having a thickness in the range of about 0.1 mil toabout 5 mil, for example about 0.1 mil to about 2 mil, or for example,about 0.5 mil to about 5 mil. The uncured epoxy film can have a curingtemperature of between about 80° C. to about 300° C.

In an embodiment, the adhesive, when properly processed in accordancewith an embodiment of the present teachings, may enable the fabricationof a high performance, low cost, high density ink jet printhead. Theadhesive is chemically resistant to hostile inks used in currentprinting applications and maintains adhesion in high-temperature,high-pressure printing conditions.

The uncured epoxy film is a B-stage, two part epoxy. As with manyepoxies, the uncured epoxy film can include an epoxy resin and an epoxycuring agent (i.e., hardener) which are mixed together to provide thefinal adhesive compound. More specifically, the uncured epoxy film cancomprise a blend of base components including at least one bisphenol Aepoxy resin, such as two bisphenol A epoxy resins, cresol novolac epoxyresin, an imidazole amine hardener, and a latent curing agent,dicyandiamide (i.e., “DICY”). The blend of the bisphenol A epoxy resins(DGEBA resins) and the cresol novolac epoxy resin coupled with thehardener and latent curing agent can be cured to provide an adhesivewith adequate thermo-oxidation resistance, good workability, long potlife, and higher heat resistance than some other adhesives.Additionally, the relatively small amount of the DICY latent curingagent present, e.g., about 2% to 3% by weight, reduces the number ofamine linkages in the cured adhesive which are otherwise susceptible tooxidative attack. The combination of resins and curing agent chemistriesand ratios provide an adhesive with an extended pot life at roomtemperature. The uncured epoxy film may also include minimal amount ofsolvent, such as 2-butoxy ethyl acetate, to provide improved ease ofhandling of the uncured epoxy film.

The uncured epoxy film can be provided in predetermined geometries. Forexample, a larger sheet of the uncured epoxy film can be cut via laserablation into smaller portions to match, for example, the geometries ofliners or plates to which the adhesive compound can be adhered to uponcuring. In an embodiment, the uncured epoxy film can have a thickness inthe range of about 0.1 mil (i.e., 0.0001 inches) and about 5 mil (i.e.,0.005 inches), or between about 0.1 mil and about 4 mil, or betweenabout 0.1 mil and about 2 mil.

A chemical structure of the cresol novolac epoxy resin may be:

wherein n represents the number of repeating segments, and is, forexample, a number of from about 1 to about 30, from about 2 to about 18,or from about 3 to about 10.

Another chemical structure of the cresol novolac epoxy resin may be:

wherein n represents the number of repeating segments, and is, forexample, a number of from about 1 to about 30, from about 2 to about 18,or from about 3 to about 10.

The curing agent used may be DICY, which has the form:

DICY is a representative latent curing agent that forms crystals whenprocessed in accordance with the present teachings. It may be used inthe form of a fine powder dispersed within the resin. The material has avery long pot life, for example 6 to 12 months. DICY cures at a hightemperature, for example from about 160° C. to about 180° C. in about 20minutes to about 60 minutes. Cured DICY resins have a good adhesivenessand are less prone to staining that some other resins. DICY may be usedin one-part adhesives, powder paints, and pre-impregnated compositefibers (i.e., “pre-pregs”).

Another co-curing agent that may be used in the uncured epoxy film isimidazole. Imidazoles are characterized by a relatively long pot life,the ability to form cured resin with a high heat deformation temperatureby thermally treating at a medium temperature (80° C. to 120° C.) for arelatively short duration, and the availability of various derivativeshaving moderate reactivity that improves workability. When used as aco-curing agent with DICY, imidazole may exhibit a better pot life, afaster curing speed, and a higher heat resistance of the cured substancethan when an adhesive is used with some other co-curing agents.

Some representative chemical structures of various imidazoles, one ormore of which may be included as a co-curing agent, include

The adhesive compound may be an uncured epoxy film. The uncured epoxyfilm can be a solid, standalone film. That is, the uncured epoxy filmcan be transported as a free film without being disposed on a substratesupport. However, to prevent multiple ones of the uncured epoxy filmfrom contacting one another, the uncured epoxy film can be transportedinterposed between a first release liner and a second release liner.

Upon curing the uncured epoxy film to form an adhesive, the adhesive canbe used to attach a first substrate to a second substrate. In anembodiment of the present teachings, the release liner is removed toexpose a first surface of the uncured epoxy film. The first surface ofthe uncured epoxy film is contacted to a surface of the first substrate,and the second release liner is removed to expose a second surface ofthe uncured epoxy film. The second surface of the uncured epoxy film iscontacted to a surface of the second substrate. The embodimentsdescribed below are with reference to a solid uncured epoxy filminterposed between a first release liner and a second release liner,although other embodiments are contemplated.

In an embodiment of the present teachings, the uncured epoxy film may becured for to adhere or bond two surfaces together using a particularprocess to apply the adhesive. The process may result in the adhesivehaving various desirable operating characteristics or properties for aprinthead fabrication application that are not found if a differentapplication process is used. A novel fabrication process has beendeveloped to enable the use of epoxy adhesive formed by curing astandalone, uncured epoxy film, for printhead interstitial bonding withlittle or no squeeze out, for example, as compared to a liquidepoxy-based adhesive, at high pressure and good bonding strength withlittle or no formation of trapped air bubbles.

The procedure for attaching two or more surfaces together by curing theadhesive compound described above may include an embodiment of thefollowing process. While the process is described with reference to theattachment of a polyimide film as a first substrate and a stainlesssteel sheet as a second substrate by curing a standalone uncured epoxyfilm for simplicity of description, it will be understood that theuncured epoxy film may be cured to form adhesive that can attach othersubstrates together, for example various metals, polyimide layers,polymers other than polyimide, and combinations thereof.

In an embodiment, the substrate surfaces that are to be bonded areprepared using a surface preparation process. The composition of thesubstrate material will depend on the application and may include metalssuch as stainless steel or aluminum, or polymers such as polyimides. Thesurface preparation can include cleaning the first and second substratesusing a solvent such as isopropyl alcohol (isopropanol, IPA) to removetrace contaminating substances such as oils and airborne particulates.

After cleaning the bonding surfaces of the substrate material with asolvent, the surface preparation may also include subjecting the bondingsurfaces to a plasma cleaning process. In an embodiment, the plasmacleaning process may include an oxygen plasma cleaning process for aduration of from about 2 minutes to about 10 minutes. The plasmacleaning process is used to further remove any contaminants from thebonding surfaces and also to roughen the substrate to increase thebonding surface area for improved adhesion.

After surface preparation, the uncured epoxy film and first substrateare prepared for tacking with the uncured epoxy film. In an embodiment,the first release liner is removed from the first surface of the soliduncured epoxy film. At least the bonding surface of the first substrateis heated to a tacking temperature of between about 40° C. and about120° C., or between about 50° C. and about 100° C., or between about 50°C. and about 60° C. This heating may be performed, for example, byheating the first substrate on a hot plate or within an oven. The entirefirst substrate may be heated, or only the bonding surface may beheated. After heating the bonding surface of the first substrate, thefirst surface of the solid uncured epoxy film is contacted with thebonding surface of the first substrate, for example by placing the firstsurface of the uncured epoxy film onto the bonding surface to tack thefirst substrate to the uncured epoxy film, thereby forming a firstsubstrate-uncured epoxy film assembly.

At the tacking temperature, the first substrate-adhesive assembly isrolled by moving a roller under pressure across either the exposedsecond release liner on the second surface of the uncured epoxy film,across a second surface (back side) of the first substrate, or both.This rolling stage assists in removing air bubbles at the interfacebetween the first surface of the adhesive and the bonding surface of thefirst substrate. In an embodiment, the roller may be rolled across thefirst substrate-uncured epoxy film assembly at a pressure, for exampleat a roller pressure of between about 1 psi and about 10 psi, or betweenabout 1 psi and about 5 psi, against the surface.

After this processing stage, the first substrate-uncured epoxy filmassembly is cooled to ambient temperature of 22° C. or less to result inthe first substrate being tacked to the uncured epoxy film. This tackprocedure serves to wet the uncured epoxy film onto the substrate toreduce or eliminate air bubbles.

Subsequently, the second release liner is removed from the secondsurface of the solid uncured epoxy film. At least the bonding surface ofthe second substrate is heated to the tacking temperature as describedabove for the first substrate, between about 40° C. and about 120° C.,or between about 50° C. and about 100° C., or between about 50° C. andabout 60° C. for a duration of between about 1 minute and about 5minutes. This heating may be performed, for example, by heating thesecond substrate on a hot plate or within an oven. The entire secondsubstrate may be heated, or only the bonding surface may be heated.After heating the bonding surface of the second substrate, the secondsurface of the solid uncured epoxy film is contacted with the bondingsurface of the second substrate, for example by placing the secondsurface of the uncured epoxy film onto the bonding surface to tack thesecond substrate to the uncured epoxy film, thereby forming a secondsubstrate-uncured epoxy film assembly.

At the tacking temperature, the second substrate-uncured epoxy filmassembly is rolled by moving a roller under pressure across the back ofthe second substrate to assist in the removal of bubbles at theinterface between the second surface of the uncured epoxy film and thebonding surface of the second substrate. In an embodiment, the rollermay be rolled across the second substrate-uncured epoxy film assembly ata pressure, for example at a roller pressure of between about 1 psi andabout 10 psi, or between about 1 psi and about 5 psi, against thesurface. After rolling the second substrate-uncured epoxy film assembly,first and second substrates are partially adhered together using thetacky uncured epoxy film, thereby forming a three layer assembly. Thethree layer assembly, including the first and second substrates and theuncured epoxy film, may be cooled to ambient temperature, or may proceeddirectly to the next processing stage without cooling, for example byramping the temperature to a partial curing temperature as describedbelow. For example, because the uncured epoxy film has a curingtemperature in the range of between about 80° C. and about 300° C., orbetween about 90° C. and about 200° C. or between about 120° C. to about190° C., the uncured epoxy film can be partially or fully cureddepending on temperature to which the uncured epoxy film is heated.While the process is described with reference to a three layer assembly,the three layer assembly may include additional substrates attached tothe three layer assembly, for example using other layers of uncuredepoxy film or other portions of the existing uncured epoxy film.

After rolling the second substrate-adhesive assembly to form the threelayer assembly, an adhesive partial curing process is performed. Thethree layer assembly may be heated to a partial curing temperature ofbetween about 80° C. and about 140° C., or between about 90° C. andabout 120° C., or between about 100° C. and about 120° C., for exampleabout 120° C. The three layer assembly may be heated, for example, on ahot plate or in an oven. Once the three layer assembly reaches thepartial curing temperature, heat is applied to the three layer assemblyfor a duration of between about 10 minutes and about 20 minutes, forexample about 15 minutes. This partial curing process is a crucial stageto decrease, minimize, or eliminate squeeze out of the adhesive compoundduring subsequent processing. If a temperature and/or duration of thispartial curing stage is insufficient or excessive, adhesive compoundsqueeze-out, over-curing of the adhesive compound, or damage to theadhesive compound components may occur.

After the partial curing process, the three layer assembly is heatedunder pressure, for example in a jet stack press, to a final bondingtemperature to complete the bonding process to polymerize the adhesivecompound (i.e., the uncured epoxy film) and thereby form a fully curedadhesive. In an embodiment, the assembly is heated to a final bondingtemperature of between about 100° C. and about 300° C., or between about150° C. and about 200° C., or between about 180° C. and about 200° C.,for example about 190° C. Once the three layer assembly reaches thefinal bonding temperature, heat is applied to the three layer assemblyfor a duration of between about 20 minutes and about 200 minutes, orbetween about 60 minutes and about 100 minutes, for example about 70minutes. During the application of the final bonding temperature, abonding pressure of between about 40 psi and about 100 psi, or at apressure of 70 psi or less, or at a maximum pressure of 70 psi, forexample a pressure of about 55 psi, is applied to the three layerassembly within the jet stack press. After curing the adhesive compoundat the final bonding temperature under the bonding pressure for theabove-stated duration, the pressure and temperature are decreased toambient levels to complete the bonding process. For purposes of thisdisclosure, a “fully cured” adhesive compound refers to an adhesive thatis sufficiently cured (i.e., polymerized and crosslinked) for use of theprinthead (e.g., more than 95% cured). A curing agent such as DICYand/or a co-curing agent such as imidazole, which may or may not bepresent in the uncured epoxy film, may or may not remain in the fullycured adhesive depending, for example, on whether the fully curedadhesive is 100% cured. In an embodiment, depending on a ratio of epoxyto curing agent in the pre-cured formulation (i.e., the uncured epoxyfilm), the curing agent such as DICY may remain in the matrix aftercuring even if the adhesive is 100% cured.

In an embodiment, epoxy adhesive formed by curing the adhesive compound(i.e., curing the uncured epoxy film) may be used, referring to FIG. 1,as the external manifold adhesive 18, the diverter attach adhesive 22,the aperture plate adhesive 48, the boss plate adhesive 70, thediaphragm adhesive 72, or generally any printhead adhesive. The epoxyadhesive may be used to physically attach any combination of one or moremetals (e.g., stainless steel, aluminum, copper, metal alloy, etc.), oneor more semiconductors (e.g., silicon, gallium arsenide, etc.), and/orone or more organic or inorganic polymers (e.g., polyimide, nylon,silicone, etc.).

During testing, it was found that a cured epoxy adhesive compoundprepared according one or more of the process embodiments describedabove demonstrated characteristics and properties well suited forprinthead applications. In one test, nozzles were drilled into anadhesive sample including the cured adhesive compound prepared accordingto an embodiment described above and evaluated for bubbles. No bubblesgreater than 20 μm were detected using this process.

Wicking or squeeze out of adhesive occurs when the cured adhesivecompound has a change in dimension of 5% or greater, which can lead toleakage of ink or bursting of the printhead during high-pressureprinting. For example, pressures within a solid ink jet printhead canreach up to 10 psi. The subject material demonstrated a squeeze out ofless than 5%. A squeeze out test was performed on an assembly includingadhesive formed by curing the uncured epoxy film wherein the adhesive isinterposed between a stainless steel layer and a polyimide layer with500 μm nozzles processed according to an embodiment described above. Thecompleted adhesive had a thickness of about 1 mil. All of the nozzleswere open subsequent to final bonding in a jet stack press. A differentprinthead adhesive prepared using the same process failed to performsatisfactorily with complete blockage of the nozzles. Additionally, thesame adhesive without the partial curing stage failed.

To provide sufficient bonding of metal to metal, metal to polyimide, orpolyimide to polyimide, an adhesive must provide a lap shear strength,regardless of the material, of greater than about 200 psi. Someadhesives minimally meet this tolerance, do not meet this tolerance, ormeet the tolerance only at room temperatures. In a lap shear samplepreparation, the stainless steel adherends were cleaned for five minutesin an ultrasonic bath of IPA, then another four minutes of detergentultrasonic cleaning, followed by a five minutes of a rinse in deionizedwater, followed by oven drying at 100° C. for 30 minutes, then a plasmaclean. The adhesive formed by curing the uncured epoxy film was bondedbetween the adherends as described above at a thickness of 1.0 mil. Foran area of 0.62 in², a maximum load was found to be 1627.2 pound-force(lbf), and the lap shear strength was found to be greater than 2600 psi(2625 psi) bonding a first substrate to a second substrate. The materialprepared according to the method described above thus demonstrated goodbonding strength for printhead applications.

Additionally, weight gain (i.e., mass uptake) of an adhesive duringexposure to harsh inks results in swelling, which can cause leakage orbursting of the printhead during high-temperature, high-pressure use. Inan embodiment of the present teachings, when exposed to gel UV ink, thecured epoxy adhesive resisted weight gain and swelling (i.e., less than0.4% weight gain) when exposed to harsh inks at 90° C. continuously for20 weeks and is thus compatible with harsh inks. In contrast, someconventional inks used in printhead fabrication show marked weightchange when exposed to harsh inks, in some cases a percent change inweight of as much as 160% after less than 1000 hours of testing.

While some epoxy adhesives are cured using high pressures, for examplepressures greater than 200 psi, the subject material may be cured atpressures of 200 psi or less, for example about 55 psi. Extremepressures are avoided where possible during printhead manufacture, asvarious printhead structures such as piezoelectric elements andelectrical circuits may be damaged during high pressure assemblyprocesses. Still, high pressures are used in conventional processes withsome conventional adhesives to improve adhesive bonding and printheadreliability.

After forming a laminated printhead structure such as the structure ofFIG. 1 using an adhesive formed by curing the uncured epoxy film, theprinthead is filled with an ink 206 (FIG. 2), for example a UV ink or apigmented ink. These inks are particularly chemically reactive withconventional epoxy adhesives applied using conventional techniques,which are exposed to the ink within the printhead. In an embodiment, theadhesive of the embodiments, such as the adhesive formed by curing theuncured epoxy film described herein, resists chemical reaction with theink, for example weight gain and swelling (mass uptake).

Additionally, in an embodiment, a storage modulus of the epoxy adhesiveformed by curing the uncured epoxy film of the embodiments is betweenabout 100 megapascals (MPa) and about 3000 MPa at a temperature of 20°C. and between about 3 MPa and about 1500 MPa at a temperature of 120°C. An embodiment of the adhesive formed by curing the uncured epoxy filmmay further have a shelf life of greater than one month at 20° C. andgreater than one year at 0° C.

To reduce ink leakage between adjacent layers, a particle size of afiller material within an adhesive should be as small as possible.Fillers within the subject material may include a plurality ofparticles, wherein a maximum diameter (or a maximum dimension in anydirection) of each of the plurality of particles is 1.0 μm or less. Forexample, the plurality of particles can include particles of varioussizes in the ranges of from greater than about 0 μm to about 1 μm, fromgreater than about 0 μm to about 0.5 μm, or from greater than about 0 μmto about 0.2 μm. Filler materials may include one or more of calciumcarbonate, silica, alumina, alumina trihydrate, barium sulfate, titania,and kaolin clay.

FIG. 2 depicts a printer 200 including a printer housing 202 into whichat least one printhead 204 including an embodiment of the presentteachings has been installed and that encases the printhead 204. Duringoperation, ink 206 is ejected from one or more printheads 204. Theprinthead 204 is operated in accordance with digital instructions tocreate a desired image on a print medium 208 such as a paper sheet,plastic, etc. The printhead 204 may move back and forth relative to theprint medium 208 in a scanning motion to generate the printed imageswath by swath. Alternately, the printhead 204 may be held fixed and theprint medium 208 moved relative to it, creating an image as wide as theprinthead 204 in a single pass. The printhead 204 can be narrower than,or as wide as, the print medium 208. In another embodiment, theprinthead 204 can print to an intermediate surface such as a rotatingdrum or belt (not depicted for simplicity) for subsequent transfer to aprint medium.

EXAMPLES Example 1 Production of Uncured Epoxy Adhesive Film

The uncured epoxy film, which can be a B-stage epoxy film, can includeformulation comprising BPA epoxy resin. Novolac resin, latent agent andother components. The B-stage epoxy film is created by blending solidresins in a solvent carrier, preparing a homogeneous liquid slurry thatcan be coated on a surface. Most of the excess solvent is thenevaporated at a temperature below the curing temperature. This processcreates a stable solid in the desired configuration, which can then becured at a later time with exposure to heat. An advantage of thisapproach is that the quenching process for stopping curing is avoided,thus eliminating a step with high variability. The B stage film createdis a solid form uncured epoxy. It has a minimum amount of residualsolvent for handling purposes and silica fillers for mechanical strengthand film formation.

Example 2 Curing of the Uncured Epoxy Adhesive Film

The uncured epoxy film was cured at 190° C. for 70 min. Differentialscanning calorimetry (DSC) results shown in FIG. 3 confirms that theuncured epoxy film was fully cured. Dynamic Mechanical Analysis (DMA)was conducted to evaluate the physical properties of the final curedepoxy adhesive and data is presented in FIG. 4. The Tg of the epoxyadhesive (i.e., the cured epoxy film) is 149° C., which is higher thanthat of a printhead operating temperature (about 40° C. for aqueous ink,85° C. for UV gel ink, and 115° C. for solid inks).

Example 3 UV Ink Compatibility Testing

Excellent ink compatibility of adhesive formed by curing the uncuredepoxy film was demonstrated by the ink soaking tests. The uncured epoxyfilm was cured at 190° C. for 70 min and then put into various inksoaking environments with results of ink soaking tests shown in FIG. 5.LancE ink and Pigmented Black are solid inks. SunJet UV is commerciallyavailable UV ink. HP latex ink and Collins dye based ink were chosen asrepresentative commercially available aqueous inks.

As shown in the ink soaking test data shown in FIG. 5, the weightchanges in Lab air, Hot Nitrogen, Pigmented Black, Sunjet UV ink, HPlatex ink and Collins ink are all within +/−2% for up to 16 weeks. Whilenot limited to any particular theory, it is believed that weight lossesobserved in LancE ink and Hot air are likely to be minor oxidationeffects from side groups breakage within the polymer network.

Other commercially available adhesives products were evaluated forcompatibility with UV inks. See Table I below. The cured adhesives weresoaked in Xerox UV gel inks. The adhesives were incompatible with the UVgel inks in terms of weight gain or dissolution upon exposure to the UVgel ink.

TABLE 1 Percent Weight Gain Adhesive Type Commercial Product (TestDuration) Poly amide-imide KS 6600 (Hitachi) 28% (14 weeks) Nitrilephenolic based TDS 668 (3M) Dissolved Epoxy-acrylic based UV 1051 (3M)68% (1 week) Modified acrylic Pyralux FR0100 68% (2 weeks) (DuPont)

Example 4 Functional Testing of Uncured Epoxy Film Example 4a Lap ShearEvaluation

Lap shear coupons were prepared using stainless steel adherends toevaluate the bonding strength of the adhesive formed by curing theuncured epoxy film. Long term aging test were initiated in various inksoaking environments. In a typical lap shear sample preparation, thestainless steel adherends were cleaned for 5 minutes in an ultrasonicbath of IPA followed by 4 minutes of detergent ultrasonic cleaning and 5minutes of a DI water rinse. The parts were oven-dried for 30 min/100°C. and then plasma cleaned. Then the adhesive was bonded between the twostainless steel adherends using the same tacking and bonding proceduredescribed above.

FIGS. 6A-6H show results of the lap shear aging results in varioussoaking environments. Data were all collected at elevated temperature of115° C. It can be seen from FIGS. 6A-6H, that bond strengths in Lab air,Hot air, Hot Nitrogen, LancE ink, Pigmented Black and Sunjet UV are verystable for up to 24 weeks. In HP latex ink, there are notable decreasesin shear strength within 8 weeks aging, however, beyond that, ˜1200 psibond strength was well maintained for up to 24 weeks. In Collins dyebased ink, initial decreases in shear strength were observed within 4weeks of aging, while no further decreases were observed by up to 16weeks aging and ˜1500 psi bond strength was well kept. Overall, adhesiveformed by curing the uncured epoxy film has much higher bond strengththan 200 psi. Additionally, over the aging process and in all thedifferent inks, high bond strengths were well maintained, indicatingthat adhesive formed by curing the uncured epoxy film has very goodbonding performance in various inks.

Example 4b Materials Burst Test Structure

Adhesive formed by curing the epoxy film was disposed in materials bursttest structure (MTS) testing; a functional testing to monitor the leaksin the bonding interfaces. The prepared MTS in both coarse and finefeatures were soaked into several aging environments, including variousinks. As shown in FIGS. 7A-7B, no failure was observed in any of theaging environments in more than 110 days.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the present teachings are approximations, thenumerical values set forth in the specific examples are reported asprecisely as possible. Any numerical value, however, inherently containscertain errors necessarily resulting from the standard deviation foundin their respective testing measurements. Moreover, all ranges disclosedherein are to be understood to encompass any and all sub-ranges subsumedtherein. For example, a range of “less than 10” can include any and allsub-ranges between (and including) the minimum value of zero and themaximum value of 10, that is, any and all sub-ranges having a minimumvalue of equal to or greater than zero and a maximum value of equal toor less than 10, e.g., 1 to 5. In certain cases, the numerical values asstated for the parameter can take on negative values. In this case, theexample value of range stated as “less than 10” can assume negativevalues, e.g. −1, −2, −3, −10, −20, −30, etc.

While the present teachings have been illustrated with respect to one ormore implementations, alterations and/or modifications can be made tothe illustrated examples without departing from the spirit and scope ofthe appended claims. For example, it will be appreciated that while theprocess is described as a series of acts or events, the presentteachings are not limited by the ordering of such acts or events. Someacts may occur in different orders and/or concurrently with other actsor events apart from those described herein. Also, not all processstages may be required to implement a methodology in accordance with oneor more aspects or embodiments of the present teachings. It will beappreciated that structural components and/or processing stages can beadded or existing structural components and/or processing stages can beremoved or modified. Further, one or more of the acts depicted hereinmay be carried out in one or more separate acts and/or phases.Furthermore, to the extent that the terms “including,” “includes,”“having,” “has,” “with,” or variants thereof are used in either thedetailed description and the claims, such terms are intended to beinclusive in a manner similar to the term “comprising.” The term “atleast one of” is used to mean one or more of the listed items can beselected. Further, in the discussion and claims herein, the term “on”used with respect to two materials, one “on” the other, means at leastsome contact between the materials, while “over” means the materials arein proximity, but possibly with one or more additional interveningmaterials such that contact is possible but not required. Neither “on”nor “over” implies any directionality as used herein. The term“conformal” describes a coating material in which angles of theunderlying material are preserved by the conformal material. The term“about” indicates that the value listed may be somewhat altered, as longas the alteration does not result in nonconformance of the process orstructure to the illustrated embodiment. Finally, “exemplary” indicatesthe description is used as an example, rather than implying that it isan ideal. Other embodiments of the present teachings will be apparent tothose skilled in the art from consideration of the specification andpractice of the disclosure herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the present teachings being indicated by the following claims.

Terms of relative position as used in this application are defined basedon a plane parallel to the conventional plane or working surface of aworkpiece, regardless of the orientation of the workpiece. The term“horizontal” or “lateral” as used in this application is defined as aplane parallel to the conventional plane or working surface of aworkpiece, regardless of the orientation of the workpiece. The term“vertical” refers to a direction perpendicular to the horizontal. Termssuch as “on,” “side” (as in “sidewall”). “higher.” “lower,” “over,”“top,” and “under” are defined with respect to the conventional plane orworking surface being on the top surface of the workpiece, regardless ofthe orientation of the workpiece.

1. An adhesive compound, comprising: an uncured epoxy film having acuring temperature between about 80° C. and about 300° C., the uncuredepoxy film comprising a cresol novolac epoxy resin, and at least onebisphenol A epoxy resin wherein the uncured epoxy film comprises athickness between about 0.1 mil and about 5 mil, and wherein a chemicalstructure of the cresol novolac epoxy resin is:

wherein n represents the number of repeating segments, and is a numberof from about 1 to about
 30. 2. The adhesive compound of claim 1,wherein the uncured epoxy film further comprises a dicyandiamide curingagent and wherein a chemical structure of the dicyandiamide curing agentis:


3. The adhesive compound of claim 1, wherein the uncured epoxy filmfurther comprises an imidazole co-curing agent and a chemical structureof the imidazole co-curing agent is:


4. The adhesive compound of claim 1, wherein the uncured epoxy filmfurther comprises a filler material comprising a plurality of particles,wherein a maximum dimension of each of the plurality of particles is 1.0μm or less.
 5. The adhesive compound of claim 4, wherein the fillermaterial comprises one or more of calcium carbonate, silica, alumina,alumina trihydrate, barium sulfate, titania, and kaolin clay.
 6. Theadhesive compound of claim 1, wherein the uncured epoxy film furthercomprises a filler material comprising a plurality of particles, whereinthe plurality of particles comprise particles having sizes in the rangeof from greater than about 0 μm to about 0.5 μm.
 7. The adhesivecompound of claim 1, wherein the thickness is between about 0.1 mil andabout 2 mil.
 8. The adhesive compound of claim 1, wherein the uncuredepoxy film, when cured, comprises an adhesive epoxy with a storagemodulus between about 100 megapascals (MPa) and about 3000 MPa at atemperature of 20° C. and between about 3 MPa and about 1500 MPa at atemperature of 120° C.
 9. The adhesive compound of claim 1, wherein theuncured epoxy film has a shelf life of greater than one month at 20° C.and greater than one year at 0° C.
 10. The adhesive compound of claim 1,wherein the uncured epoxy film can be transported as a standalone filmwithout being disposed on a substrate support.
 11. A method, comprising:forming an epoxy adhesive by curing an adhesive compound, wherein theadhesive compound comprises an uncured epoxy film having a thicknessbetween about 0.1 mil and about 5 mil and a curing temperature betweenabout 80° C. and about 300° C., wherein the uncured epoxy film comprisesa cresol novolac epoxy resin, and at least one bisphenol A epoxy resin,wherein a chemical structure of the cresol novolac epoxy resin is:

wherein n represents the number of repeating segments, and is a numberof from about 1 to about
 30. 12. The method of claim 11, wherein theslurry composition further comprises a dicyandiamide curing agent andwherein a chemical structure of the dicyandiamide curing agent is:


13. The method of claim 11, wherein the slurry composition furthercomprises an imidazole co-curing agent and a chemical structure of theimidazole co-curing agent is:


14. The method of claim 11, wherein the uncured epoxy film furthercomprises a filler material comprising a plurality of particles, whereina maximum dimension of each of the plurality of particles is 1.0 μm orless.
 15. The method of claim 14, wherein the filler material comprisesone or more of calcium carbonate, silica, alumina, alumina trihydrate,barium sulfate, titania, and kaolin clay.
 16. The method of claim 11,wherein the uncured epoxy film further comprises a filler materialcomprising a plurality of particles, wherein the plurality of particlescomprise particles having sizes in the range of from greater than about0 μm to about 0.5 μm.
 17. The method of claim 11, wherein the thicknessis between about 0.1 mil and about 2 mil.
 18. The method of claim 11,wherein a storage modulus of the epoxy adhesive is between about 100megapascals (MPa) and about 1500 MPa at a temperature of 20° C. andbetween about 3 MPa and about 700 MPa at a temperature of 120° C. 19.The method of claim 11, wherein the epoxy adhesive has a shelf life ofgreater than one month at 20° C. and greater than one year at 0° C. 20.The method of claim 11, wherein the uncured epoxy film can betransported as a standalone film without being disposed on a substratesupport.